Multi-level modulation has been actively applied to an optical communication system in view of enhancement in capacity and improvement in frequency use efficiency. So as to realize a more flexible system configuration, there is disclosed a transmission method involving changing a modulation scheme (transmission rate or transmission bandwidth) in an adaptive manner with use of multi-level modulation (see, for example, Non Patent Literature 1). In Non Patent Literature 1, each Mach-Zehnder (MZ) modulator is driven by binary signals. Thus, a normal binary modulation scheme can be used for bias control of each MZ modulator. In this case, however, the number of modulators needs to be increased as the multi-level degree increases. As a result, there is a problem in that the configuration of the optical transmitter is complicated.
Further, so as to enhance transmission performance, there is known a method involving performing the inverse arithmetic process of a transmission channel transfer function by using digital signal processing on the transmitter side and generating an optical signal with use of an analog waveform obtained through a high speed D/A converter (see, for example, Non Patent Literature 2). In Non Patent Literature 2, an optical waveform is generated based on an analog signal obtained by a D/A converter. In this method, a signal peak-to-average power ratio (PAPR) varies depending on transmission conditions, and it is therefore necessary to appropriately manage a DC bias adjustment in driving of a MZ modulator. As a bias control method for a modulator driven with an arbitrary analog waveform, a control method using a dither signal has hitherto been proposed (see, for example, Patent Literature 1 and Patent Literature 2). Further, it is known that the bias control has such characteristics that the polarity thereof changes due to a difference of an average drive amplitude caused by PAPR variations. To avoid this, there is disclosed a technology of controlling the drive amplitude in accordance with the PAPR to perform stable bias control (see, for example, Patent Literature 1 and Non Patent Literature 3).
In the case of supporting a plurality of multi-level modulation schemes, a multi-level signal generation technology using D/A conversion is known as a method for achieving a simpler transmitter configuration by suppressing an increase in number of MZ modulators caused by an increase in multi-level degree (see, for example, Patent Literature 3 and Non Patent Literature 4). Mapping of transmission data to a multi-level signal can be easily realized by multi-level modulation with use of a lookup table described in Patent Literature 4, for example.